1. Field of the Invention
The present invention relates to driving circuits for switch mode radio frequency (RF) power amplifiers.
2. State of the Art
Battery life is a significant concern in wireless communications devices such as cellular telephones, pagers, wireless modems, etc. Radio-frequency transmission, especially, consumes considerable power. A contributing factor to such power consumption is inefficient power amplifier operation. A typical RF power amplifier for wireless communications operates with only about 10% efficiency. Clearly, a low-cost technique for significantly boosting amplifier efficiency would satisfy an acute need.
Furthermore, most modern digital wireless communications devices operate on a packet basis. That is, the transmitted information is sent in a series of one or more short bursts, where the transmitter is active only during the burst times and inactive at all other times. It is therefore also desirable that control of burst activation and deactivation be controlled in an energy-efficient manner, further contributing to extended battery life.
Power amplifiers are classified into different groups: Class A, Class B, Class AB, etc. The different classes of power amplifiers usually signify different biasing or load conditions. In designing an RF power amplifier, there is usually a trade-off between linearity and efficiency. The different classes of amplifier operation offer designers ways to balance these two parameters.
Generally speaking, power amplifiers are divided into two different categories, linear and non-linear. Linear amplifiers (e.g. Class A amplifiers and Class B push-pull amplifiers), maintain high linearity, resulting in faithful reproduction of the input signal at their output since the output signal is linearly proportional to the input signal. In non-linear amplifiers (e.g. single-ended Class B and Class C amplifiers), the output signal is not directly proportional to the input signal. The resulting amplitude distortion on the output signal makes these amplifiers most applicable to signals without any amplitude modulation, which are also known as constant-envelope signals.
Amplifier output efficiency is defined as the ratio between the RF output power and the input (DC) power. A major source of power amplifier inefficiency is power dissipated in the transistor. A Class A amplifier is inefficient since current flows continuously through the device, whether or not there is an output signal. Conventionally, efficiency is improved by trading-off linearity for increased efficiency. In Class B amplifiers, for example, biasing conditions are chosen such that the output signal is cut off during half of the cycle unless the opposing half is provided by a second transistor (push-pull). As a result, the waveform will be less linear. The output waveform may still be made sinusoidal using a tank circuit or other filter to filter out higher and lower frequency components.
Class C amplifiers conduct during less than 50% of the cycle, in order to further increase efficiency; i.e., if the output current conduction angle is less than 180 degrees, the amplifier is referred to as Class C. This mode of operation can have a greater efficiency than Class A or Class B, but it typically creates more distortion than Class A or Class B amplifiers. In the case of a Class C amplifier, there is still some change in output amplitude when the input amplitude is varied. This is because the Class C amplifier operates as a controlled current source--albeit one that is only on briefly--and not as a switch.
The remaining classes of amplifiers vigorously attack the problem of power dissipation within the transistor, using the transistor merely as a switch. The underlying principle of such amplifiers is that a switch ideally dissipates no power, for there is either zero voltage across it or zero current through it. Since the switch's V-I product is therefore always zero, there is (ideally) no dissipation in this device. A Class E power amplifier uses a single transistor, in contrast with a Class D power amplifier, which uses two transistors
In real life, however, switches are not ideal. (Switches have turn on/off time and on-resistance.) The associated dissipation degrades efficiency. The prior art has therefore sought for ways to modify so-called "switch-mode" amplifiers (in which the transistor is driven to act as a switch at the operating frequency to minimize the power dissipated while the transistor is conducting current) so that the switch voltage is zero for a non-zero interval of time about the instant of switching, thereby decreasing power dissipation. The Class E amplifier uses a reactive output network that provides enough degrees of freedom to shape the switch voltage to have both zero value and zero slope at switch turn-on, thus reducing switching losses. Class F amplifiers are still a further class of switch-mode amplifiers. Class F amplifiers generate a more square output waveform as compared to the usual sinewave. This "squaring-up" of the output waveform is achieved by encouraging the generation of odd-order harmonics (i.e., x3, x5, x7, etc.) and suppressing the even-order harmonics (i.e., x2, x4, etc.) in the output network.
FIG. 1 shows an example of a known Class E power amplifier, described in U.S. Pat. No. 3,919,656, incorporated herein by reference. An RF input signal is coupled over a lead 1 to a driver stage 2, the latter controlling the active device 5 via a signal coupled over a lead 3. The active device 5 acts substantially as a switch when appropriately driven by the driver 2. The output port of the active device is therefore represented as a single-pole single-throw switch 6. Connected across the switch 6 is the series combination of a DC power supply 7 and the input port of a load network 9. The output port of the load network 9 is connected to the load 11. As the switch 6 is cyclically operated at the desired AC output frequency, DC energy from the power supply 7 is converted into AC energy at the switching frequency (and harmonics thereof).
The arrangement of FIG. 1, although it is capable of achieving high conPATENT version efficiency, suffers from the disadvantage that large voltage swings occur at the output of the active device, due to ringing. This large voltage swing, which typically exceeds three times the supply voltage, precludes the use of the Class E circuit with certain active devices which have a low breakdown voltage.
Furthermore, the driving circuit in the RF amplifier typically includes a matching network consisting of a tuned (resonant) circuit. Referring to FIG. 2, in such an arrangement, an RF input signal is coupled to a driver amplifier, typically of Class A operation. An output signal of the driver amplifier is coupled through the matching network to a control terminal of the switching transistor, shown in FIG. 2 as an FET. As with design of the load network of FIG. 1, proper design of the matching network is not an easy matter.